Presidential hopeful Rick Santorum is drawing more scrutiny as his poll numbers rise, and so is the increasingly lucrative super PAC backing the former Pennsylvania Senator.

The pro-Santorum Red, White and Blue Fund raised $2 million in January, more than twice what the political action committee took in during all of 2011, according to public disclosure reports filed this week with the Federal Election Commission. But the PAC also faces calls for a Justice Department investigation by a watchdog group and has returned a banned $50,000 foreign donation from a London securities firm.

The super PAC has engaged in illegal coordination with the Santorum presidential campaign, alleges a letter sent to Attorney General Eric Holder today by Democracy 21. The watchdog group leveled similar allegations last week at the super PACs backing GOP presidential contender Mitt Romney and President Barack Obama.

The letter cites news reports suggesting that Foster Friess, founder of the investment management firm Friess Associates, was engaged in discussions with the super PAC about the nature of its ads at the same time as he was campaigning with Santorum as a member of the candidate’s entourage. Ushered in by the Supreme Court’s 2010 Citizens United v. FEC ruling, super PACs may raise and spend unrestricted money, but only if they act independently from candidates.

Friess, who apologized last week for poorly received jokes involving women and contraception, has given $1 million to the Red, White and Blue super PAC since its inception last year, public records show.

The Santorum super PAC’s other top donor is William Dore, president of Dore Energy Corp. in Louisiana, who gave $1 million last month, according to this week’s FEC disclosures. The filings also show that the Red, White and Blue Fund returned a $50,000 donation from Liquid Capital Markets of London, an illegal foreign source, the Sunlight Foundation reported.

This week’s FEC filings underscore the increasingly dominant role that super PACs and their billionaire backers are playing in the GOP presidential primary.

Mega-donors giving six- and seven-figure contributions make up the bulk of the money raised so far by GOP presidential super PACs, which in January pulled in more collectively than the candidates they back.

The $10 million that casino mogul Sheldon Adelson and his wife, Miriam, gave to the super PAC Winning Our Future, which backs former Speaker Newt Gingrich (Ga.), constitutes the vast majority of the money raised by that PAC.

PayPal co-founder and San Francisco investor Peter Thiel donated $1 million of the $2.4 million raised last month by Endorse Liberty, the super PAC supporting Rep. Ron Paul (R-Texas).

Restore Our Future, the super PAC backing Mitt Romney, collected two dozen contributions last month totaling $100,000 or more apiece from wealthy CEOs, investors and financial services industry executives. These included $500,000 each from Alliance Coal President Joseph Craft and from 4Life Research CEO David Lisonbee.

A substantial number of the big donors underwriting super PACs also are giving the maximum to the candidates’ campaigns, according to a report released today by a trio of watchdog groups. In 2011, 84 percent of the donors to the Restore Our Future super PAC — 172 donors — had also given the maximum individual contribution to Romney’s campaign, according to the report released by the Campaign Legal Center, the Center for Responsive Politics and Democracy 21.

Five of the 18 individuals who contributed to the pro-Gingrich Winning Our Future PAC in 2011 also gave the maximum to the Gingrich campaign, the report found. Fifteen of the 55 individual donors to the pro-Obama super PAC Priorities USA Action had also donated the maximum to the Obama campaign.

The pro-Paul super PAC Endorse Liberty had only one donor who backed both the PAC and the campaign. As for the pro-Santorum Red, White and Blue Fund, the report found that nine of the PAC’s 14 individual donors also gave the maximum to Santorum’s campaign. The double-giving illustrates the extent to which super PACs are operating as “shadow” campaign committees of the candidates they back, the report concludes.